Boris Johnson will announce which campaign he backs at 10pm Credit: PA

Boris Johnson is expected to announce tonight which side of the "Brexit" debate he supports.

It is widely expected the London mayor will come down on the side of Britain leaving the European Union when he reveals his decision in his Daily Telegraph column, which will be tweeted at 10pm.

Today his sister insisted Mr Johnson was not "milking" the announcement for "maximum publicity."

Writer and broadcaster Rachel Johnson said her brother had to make a "very, very hard choice."

"This is an enormously complicated decision for everybody, not just Boris... and his participation in either camp is going to be very significant, and this is why he's taking so long to decide," she told Sky News.